Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid-oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Hydrogen typically is derived by catalytically reforming hydrocarbons such as gasoline in the presence of limited oxygen.
A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. Therefore, in practice it is known to stack together, in electrical series, a plurality of cells.
An SOFC system requires stack temperatures above about 750° C. for electricity generation. Optimal steady-state operating temperatures may be 850° C. or even higher. A known problem in the art is how to raise the stack elements to at least the threshold temperature at start-up. It is known to use the reformate being supplied to the stack as a heat transfer agent. At air/fuel ratios at or near stoichometric partial oxidation, or POX, a POX reformer operates at a temperature of about 700° C. to about 950° C. However, if the fuel/air mixture provided to the reformer is made fuel-lean (that is, more combustion in the reformer), the output temperature can be raised even further. Operating in this mode, the reformer will produce more water and carbon dioxide, and less methane.
Two problems are encountered when operating the reformer at or near stoichometric POX. First, the reformate stream can still include some residual non-reformed hydrocarbons, which can cause coking of the anodes in the stack, a highly undesirable condition. Second, elevated temperatures within the reformer can shorten the life of, or directly damage, the catalytic elements in the reformer. Operating the reformer in a more fuel-lean condition reduces methane and hydrocarbons and increases water in the reformate stream, both of which reduce the potential of coking of the anodes. However, operating in this mode increases the reformate temperature, further reducing the life of the catalytic elements.
What is needed is a means for fueling the SOFC stack without coking of the anodes.
What is further needed is a means for providing sufficient water and reduced methane to the stack without causing overheating of the reformer catalyst.
It is a principal object of the present invention to fuel an SOFC stack without requiring overheating of the reformer catalyst.